1. Field of the Invention
The present invention relates to an optical communication method or system, and more particularly, to the transmission of an MPEG-2 TS (Moving Picture Experts Group-2 Transport Stream).
2. Description of the Related Art
Digital broadcasting that employs an MPEG-2 TS for video/audio data transmission has recently rapidly spread world wide. MPEG-2 has a variable bit rate (VBR) property. Moreover, it performs data compression using an algorithm which refers to the past frame to compress only the changed part into a stream data. For example, a low mobility image captured in a studio is compressed at a low bit rate. Further, a high mobility image having a large amount of frame change, captured at a sports game, etc., is compressed at a sharply increased bit rate.
However, in many cases, MPEG-2 data having a VBR property is transmitted in a constant bit rate (CBR) fashion to viewers. However, bandwidth prediction and assurance issues are taking into consideration.
For example, in Korea, regular terrestrial digital broadcasting has been provided to the metropolitan areas since 2001. This digital broadcasting employs an 8-level vestigial sideband (8-VSB) mode according to the American advanced television systems committee (ATSC) specifications. In particular, MPEG-2 video/audio data is transmitted at 19.39 Mbps in a CBR fashion via a radio frequency (RF) spectrum band of 6 MHz for one channel. In such terrestrial or cable broadcasting, MPEG-2 data is modulated and demodulated in an 8-VSB or QAM mode, respectively, before and after it is transmitted. In addition, MPEG-2 data transmission is performed mainly in a CBR fashion.
In the meantime, a time division multiplexing (TDM) scheme is used in a backbone network (such as a synchronous digital hierarchy/synchronous optical network (SDH/SONET), or in a plesiochronous digital hierarchy (PDH) network) provided between broadcast stations, program providers (PP) or system operators (SO).
Thus, if data received in a CBR fashion is transmitted without alteration to the TDM-based backbone network, it may cause an excessive and needless waste of bandwidth.
FIGS. 1a and 1b illustrate a conventional connection of an MPEG-2 TS to a backbone network.
As shown in FIG. 1a, an MPEG-2 TS of a CBR or VBR mode received through a DVB-ASI input is converted into a CBR mode through a Remux 101. It is then transmitted to a backbone network through a network transmission unit 102. As shown in FIG. 1b, a received MPEG-2 TS of a VBR or CBR mode is directly transmitted to the backbone network through the network transmission unit 102.
The Remux 101 shown in FIG. 1a is digital broadcasting equipment capable of changing the bit rate of an MPEG-2 TS. It is frequently used in a broadcast station headend. However, it is impossible to economically implement the Remux 101 in a practical digital cable broadcasting system, for example, with tens to hundreds of channels. Moreover, the cost of this is very high, for example, $50,000. In order to match or adapt an MPEG-2 TS into an SDH/SONET or an ATM network as shown in FIG. 1a, the bit rate of the MPEG-2 TS must be checked if it is in a range that can be accompanied by the network into which the MPEG-2 TS will be matched. Further, the bit rate must be forced through the Remux 101 to be reduced, if the bit rate exceeds the range. In addition, for transmission through backbone networks, the data must be manipulated according to the respective standards of the backbone networks, for example, in the network transmission unit 102 shown in FIGS. 1a and 1b. 
A received MPEG-2 TS of a CBR mode is transmitted to the backbone network. If unnecessary null packets, included in the MPEG-2 TS for the CBR, are also transmitted, this leads to an unnecessary use of bandwidth of the backbone network. This, in turn, causes a waste of bandwidth. For example, when an MPEG-2 TS is adjusted to a CBR for ATSC terrestrial transmission at 19.39 Mbps or for other uses, null packets may/are used to fill in the MPEG-2 TS only for unifying the bit rate. These are transmitted through a TDM-based transmission network and cause an unnecessary waste of bandwidth. Thus, the economic burden of the system is increased.
Terrestrial digital broadcasting can be carried out even when cable broadcasting has not yet been digitalized (as in Korea) or when not all broadcasting equipment is implemented for supporting HD broadcasting at 20 Mbps to 27 Mbps levels. In these situations, the data is received by SD-level broadcasting at about 3 Mbps. It is then broadcasted by HD-level broadcasting using null packets. This causes problems in that the amount of unnecessary null packets filled in the data is 5 to 8 times larger than real data that is transmitted.
When an MPEG-2 TS is transmitted through a TDM-based backbone network, there is thus a need for a device capable of transmitting the desired broadcast information without loss. In addition, the device should not cause bandwidth waste in the TDM-based backbone network.